1. Field of the Invention
The present invention relates generally to shaft seals and, more particularly, is concerned with a method of treating a coating on seal surface in a nuclear reactor coolant pump.
2. Description of the Prior Art
In pressurized water nuclear power plants, a reactor coolant system is used to transport heat from the reactor core to steam generators for the production of steam. The steam is then used to drive a turbine generator. The reactor coolant system includes a plurality of separate cooling loops, each connected to the reactor core and containing a steam generator and a reactor coolant pump.
The reactor coolant pump typically is a vertical, single stage, centrifugal pump designed to move large volumes of reactor coolant at high temperatures and pressures, for example 550 degrees F. and 2500 psi. The pump basically includes three general sections from bottom to top--hydraulic, shaft seal and motor sections. The lower hydraulic section includes an impeller mounted on the lower end of a pump shaft which is operable within the pump casing to pump reactor coolant about the respective loop. The upper motor section includes a motor which is coupled to drive the pump shaft. The middle shaft seal section includes three tandem sealing assemblies--lower primary, middle secondary and upper tertiary sealing assemblies. The sealing assemblies are located concentric to, and near the top end of, the pump shaft. Their combined purpose is to mechanically contain the high positive pressure coolant of the reactor coolant system from leakage along the pump shaft to the containment atmosphere during normal operating condition. Representative examples of pump shaft sealing assemblies known in the prior art are the ones disclosed in U.S. Patents to MacCrum (3,522,948), Singleton (3,529,838), Villasor (3,632,117), Andrews et al (3,720,222), Boes (4,275,891), Jenkins (4,690,612) and Quinn (4,693,481) and in the first three patent applications cross-referenced above, all of which are assigned to the same assignee as the present invention.
Historically, the pump shaft seals constitute the main problem area for the reactor coolant pumps and significantly contribute to the utilization factor in nuclear power plants. The seals must be capable of breaking down the high system pressure (about 2500 psi) safely. The tandem arrangement of the three seals is used to break down the pressure, with the lower primary seal absorbing most of the pressure drop (approximately 2250 psi). The lower primary sealing assembly is the main seal of the pump. It is typically a hydrostatic, "film-riding" , controlled leakage seal whose primary components are an annular runner which rotates with the pump shaft and a non-rotating seal ring which remains stationary with the pump housing Whereas the components of the lower primary sealing assembly are not intended to contact or rub together, corresponding components of the middle and upper sealing assemblies, a rotating runner and non-rotating seal ring, provide contacting or rubbing seals.
Heretofore, the runner components of the rub-type sealing assemblies (the middle secondary and upper tertiary sealing assemblies) have been composed of a stainless steel substrate having an outer coating of chromium carbide on the surface of the runner components which rubs against the seal ring. The coating is formed by depositing chromium carbide powder on the stainless steel substrate using a detonation gun technique. Bonding between the coating and the substrate is achieved purely by mechanical impact forces when the powdered chromium carbide is impinged onto the substrate. The density of the coating thus applied is typically significantly less than 100% of theoretical.
The chromium carbide coating thus formed has proven to be less than satisfactory. Blistering has been observed to occur on the chromium carbide coated runners. The blistering is caused by contact with the nuclear water chemistry employed in nuclear reactors. This liquid penetrates through the pores of the chromium carbide coating to the stainless steel/coating interface creating a electrochemical cell and resultant corrosion. Hydrogen gas formation caused by the corrosive mechanism then results eventually in a spalling, or blistering, of the coating's surface. Thus, the blistering is attributed to the inherent porosity heretofore present in the coating and the lack of optimum bonding at the stainless steel/coating interface.
Consequently, a need exists for an effective way to prevent corrosion of the rubbing surfaces of the reactor coolant pump sealing assembly so as to improved the reliability thereof.